1. Field of the Invention
This invention relates to an expandable downhole tubular. Particularly, but not exclusively, the present invention relates to an expandable downhole tubular including a filter for removing solid particles from well fluid downhole.
2. Description of the Related Art
In the oil and gas exploration and production industry, boreholes are drilled into hydrocarbon bearing formations to recover well fluids. In well bores where a liquid, for example, oil, passes from a surrounding formation into the well bore, the liquid will often carry entrained sand particles. If this sand is permitted to pass into the well bore a number of problems may arise, including an increased likelihood of the well bore becoming blocked or restricted, and the sand may cause downhole tools to stick or jam, or wear prematurely. Accordingly, it is preferred that the sand particles are retained in the formation. This is achieved by providing screens or a filter around the casing or production tubing.
International patent publication No. WO97/17524(Shell) discloses a radially expandable assembly in which overlapping filter sheets are sandwiched between inner expandable support tubing and outer expandable protective tubing, the expandable tubing featuring large numbers of overlapping longitudinal slots. Typically, four filter sheets are provided and are welded to the inner support tubing along an axis of each sheet, and the sheets overlap in a circumferential direction. When an expansion tool is forced through the assembly, the inner and outer tubing is expanded radially, the slots extending to form diamond-shaped openings. The initial degree of overlap between the screens is selected such that, although the screens move circumferentially relative to one another during expansion, the edges of the screens remain in overlapping relation.
Whilst the performance of such assemblies in the downhole environment has been shown to be effective, assemblies including such tubing are difficult and time-consuming to manufacture, increasing the cost of the end product.
An alternative sandscreen assembly incorporates a filter sheet comprising a woven mesh strip helically wound onto an inner support tubing and secured to the inner tubing by welding along the warp wires, which define the edges of the strip. A tubular mesh is thus formed around the entire circumference of the inner support tubing.
When the assembly is expanded, the filter screen expands circumferentially by relative rotation of the warp and weft wires. Like the assembly of the Shell disclosure, it is believed the welding required to secure the filter screen to the inner support tube will be time-consuming and expensive.
European patent publication No. EP1152120 (Halliburton Energy Services, Inc) discloses an expandable well screen, which, in one embodiment, comprises a base pipe with interconnected circumferentially alternating filtering portions and expansion portions outwardly overlying the base pipe. The filtering portions each include an elongated strip of filtering media and a shroud strip overlying the filtering media. The expansion portions are of a deformable material and include longitudinally extending corrugations. The expansion portions lengthen circumferentially relative to the base pipe on expansion by opening out of the corrugations.
The well screen of EP1152120 is a complex assembly incorporating a range of components having different material properties, and expansion of the base pipe will cause a permanent plastic deformation of the expansion portions. In use, there will be a degree of elastic recovery of the well screen post expansion. However, if elastic recovery of the expansion portions is less than that of the base pipe, this will tend to cause a separation between the expansion/filtering portions and the base pipe post expansion, which is generally undesired. Additionally, if there is no connection between the expansion/filtering portions and the base tube, the likelihood of separation post expansion is increased.
Furthermore, the expansion portions, which are a critical part of the well screen, are provided radially outwardly of the base pipe and are unprotected and thus liable to be damaged, for example, through contact with a borehole wall during run-in and prior to expansion.
Also, manufacture of the well screen will be complicated by the requirement to avoid plastically deforming the expansion portions during fitting of the expansion/filtering portions to the base pipe.